Allogenic contact caves in Central East Sardinia (Italy): their speleogenesis and evolution

Several important cave systems in Central East Sardinia develop, at least partially, along the contact between the granite or phyllite basement rocks and the covering fractured Middle Jurassic sediments. The latter are composed of layers of clays, marls, conglomerates and sands of variable thickness (generally some metres) followed by sandy dolostones, becoming increasingly carbonatic moving upwards in the sequence. These karst systems are fed by allogenic streams with flow rates ranging from a couple to some thousand of L s-1. Parts of these vadose passages are sometimes developed in the insoluble basement rocks for some metres. In the summer of 2010 cavers of the Centro Speleologico Cagliaritano have discovered a new cave, Gutturu ‘e Murgulavò (Baunei), that hosts some passages entirely excavated in granitic rocks. The typical morphology of all these caves is characterised by very large and rather low chambers, which floor is masked by important graviclastic deposits. Locally the Palaeozoic bedrock is exposed along the underground streams. Speleogenesis of these cave systems is very much relied upon weathering of the basement rocks and their successive erosion by flood waters, rather than on dissolution of the carbonate rocks. Mineralogical analyses of the weathering products at the contact between Palaeozoic rocks and Mesozoic sediments have revealed the presence of montmorillonite, kaolinite, chlorite and illite, typical weathering products of granitic rocks. Dissolution, however, plays an important role in the first stages of speleogenesis, dissolving the carbonate cement in the basal sandy dolostone beds, thus leaving loose sands that can readily be eroded by running waters. Once a critical dimension in void is reached, cave formation occurs very rapidly, leading to huge chambers in some thousands of years.